Japanese Laid-open Patent Publication No. 2005-223944, for example, discusses an optical transmission system in which, in communication which uses optical fiber, control information is transmitted by adding an overhead bit to a main signal, and the main signal is demodulated at the reception side. In addition, U.S. Pat. No. 7,580,632, for example, discusses a method in which, in communication which uses optical fiber, a data signal and a path identification (ID) are transmitted by performing modulation using a tone according to the path ID with respect to an optical data signal.
In addition, Xinhui Zhao, Yong Yao, Yunxu Sun, and Chao Liu, “Circle Polarization Shift Keying With Direct Detection for Free-Space Optical Communication”, Journal of Optical Communications and Networking, Vol. 1, Issue 4, Sep. 1, 2009, pp. 307-312, for example, discusses a method in which, in free space optical communication, information is detected using a reception side polarization beam splitter by carrying information, which is different from a main signal, on the polarization wave itself by changing the status of polarization (polarization state) of single polarization. In addition, Mark D. Feuer, Vinay A. Vaishampayan, V. Mikhailov, and P. Westbrook, “Digital Lightpath Label Transcoding for Dual-Polarization QPSK Systems”, OFC/NFOEC 2011, JWA28, for example, discusses a method in which, in optical fiber communication where a main signal is transmitted by polarization multiplexing, the polarization state is changed by biasing the frequency of modulation data in the transmitted light, and the change in the polarization state is read using a reception side polarimeter.
However, in the techniques in the related art, in free space optical communication, there is a problem in that it is difficult to stably transmit data by superimposing control information which indicates a demodulation method or a decoding method of the data, without adding an optical component. For example, when the control information is superimposed and transmitted by performing intensity modulation with respect to an optical data signal, it is difficult to receive control information at the reception side, because loss of an optical signal which is intensity modulated is large in a free space. In addition, an optical component such as a polarizer is desirable at the reception side when control information is superimposed and transmitted by changing the polarization state.